Mechanical Theory of Heat to the Steam Engine. 181 



sorbs only so much heat as is consumed in doing the external ivork 

 performed, and that we find in this way many values which, at 

 the higher temperatures at least, deviate considerably from the 

 laws of Gay Lussac and Mariotte. 



This view of the behavior of steam was not shared at that 

 time even by authors who occupied themselves specially with 

 the mechanical theory of heat. W . Thomson in particular con- 

 tested the point. He found — even a year later in a memoir laid 

 before the Eoyal Society of Edinburgh— -in this result, only a proof 

 of the improbability of my collateral assumption. More recently 

 however, he has himself, associated with J. P. Joule, undertaken 

 to test the correctness of this assumption experimentally. They 

 have in fact found by a series of well devised experiments con- 

 ducted upon a large scale, that the assumption is so nearly correct 

 for the permanent gases examined by them, namely, atmospheric 

 air and hydrogen, that the variations may in most calculations be 

 neglected. They found, however, greater variations for the non- 

 permanent gas, carbonic acid, which they also studied. This 

 corresponds entirely with the remark, which I added to the first 

 mention of the assumption, that it is probably true for every gas 

 precisely in the degree in which the laws of Mariotte and Gray 

 Lussac find their application to the same gas. In consequence of 

 these experiments, Thomson has now also calculated the volume 

 of saturated steam in the same way as myself. I believe there- 

 fore that the correctness of this mode of calculation will gradu- 

 ally be more and more fully recognized by other physicists also. 



3. These two examples will suffice to shew that the fundamen- 

 tal principles of the former theory of the steam engine have 

 undergone such important changes through the mechanical the- 

 ory of heat that a new investigation of the subject is necessary. 



In the present memoir I have made the attempt to develop the 

 principles of a calculation of the work of the steam engine, cor- 

 responding with the mechanical theory of heat, in which however 

 I have confined myself to the usual forms of the steam engine 

 without at present entering upon the more recent attempts — cer- 

 tainly well worthy of consideration — to apply steam in an over- 

 heated state. 



In setting forth this investigation I shall only suppose as known 

 my last published memoir* "On an altered form of the second 

 principal theorem of the mechanical theory of heat." It is true 

 that it will in this way be necessary to deduce a second time in a 

 somewhat different manner some results which are no longer new, 

 but which were obtained at an earlier period by other writers or 

 by myself; I believe however, that this repetition will be justi- 

 fied by the greater unity and clearness of the whole. 

 • I shall refer in the proper places to the papers in which these 

 results were first communicated, as far as they are known to me. 



* Pogg. Ann., xciii, 481. 



